Reverse osmosis and nanofiltration membranes are used to separate dissolved or dispersed materials from feed streams. The separation process typically involves bringing an aqueous feed solution into contact with one surface of the membrane under pressure so as to effect permeation of the aqueous phase through the membrane while permeation of the dissolved or dispersed materials is prevented.
Both reverse osmosis and nanofiltration membranes typically include a thin film discriminating layer fixed to a porous support, collectively referred to as a “composite membrane.” Ultrafiltration and microfiltration membranes may also have a composite arrangement. The support provides physical strength but offers little resistance to flow due to its porosity. On the other hand, the discriminating layer is less porous and provides the primary means of separation of dissolved or dispersed materials. Therefore, it is generally the discriminating layer which determines a given membrane's “rejection rate,” i.e., the percentage of the particular dissolved material (i.e., solute) rejected, and “flux,” i.e., the flow rate per unit area at which the solvent passes through the membrane.
Membrane manufacturers optimize the discriminating layer for a desired combination of solvent flux and solute rejection, while also optimizing the porous support layer for maximum strength and compression resistance combined with a minimum resistance to permeate flow. In theory, a large variety of chemical compositions could be formed into thin barrier layers, however, only a few polymers offer the right combination of flux and solute rejection to generate commercially attractive reverse osmosis or nanofiltration membranes. Reverse osmosis membranes and nanofiltration membranes vary from each other with respect to their degree of permeability to different ions and compounds.
Reverse osmosis membranes are relatively impermeable to virtually all ions, including sodium and chlorine ions. Therefore, reverse osmosis membranes are widely used for the desalination of brackish water or seawater to provide relatively non-salty water for industrial, commercial, or domestic use, because the rejection rate of sodium and chlorine ions for reverse osmosis membranes is usually from about ninety-five (95) to about one hundred (100) percent.
Nanofiltration membranes are usually more specific for the rejection of ions including radium, magnesium, calcium, sulfate, and carbonate. In addition, nanofiltration membranes can be impermeable to organic compounds having molecular weights above about two hundred (200) Daltons. Additionally, nanofiltration membranes can have higher fluxes at comparable pressures than reverse osmosis membranes. These characteristics render nanofiltration membranes useful in such diverse applications as the “softening” of water and the removal of pesticides from water. As an example, nanofiltration membranes can have a sodium chloride rejection rate of from about zero (0) to about ninety-five (95) percent but have a relatively high rejection rate for salts such as magnesium sulfate and in some cases organic compounds such as atrazine.
Some membranes can be useful for reverse osmosis and nanofiltration applications by including a polyamide discriminating layer. The polyamide discriminating layer for reverse osmosis membranes is often obtained by an interfacial polycondensation reaction between a polyfunctional amine monomer and a polyfunctional acid halide monomer as described in, for example, U.S. Pat. No. 4,277,344. The polyamide discriminating layer for nanofiltration membranes can be obtained via an interfacial polymerization between a piperazine, a cyclohexane bearing at least two reactive amine or aminoalkyl groups, or a piperidine bearing at least one reactive amine or aminoalkyl group and a polyfunctional acid halide as described in U.S. Pat. Nos. 4,769,148 and 4,859,384. Another way of obtaining polyamide discriminating layers suitable for nanofiltration is via the methods described in, for example, U.S. Pat. Nos. 4,765,897; 4,812,270; and 4,824,574. These patents describe changing a reverse osmosis membrane, such as those of U.S. Pat. No. 4,277,344, into a nanofiltration membrane.
Composite polyamide membranes can be prepared by coating a porous support with a polyfunctional amine monomer, for example, from an aqueous solution. Although water is a preferred solvent, non-aqueous solvents may be utilized, such as acetyl nitrile and dimethylformamide (DMF). A polyfunctional acid halide monomer can subsequently be coated on the support, for example, from an organic solution. Although no specific order of addition is necessarily required, the amine solution can be coated first on the porous support followed by the acid halide solution. Although one or both of the polyfunctional amine and acid halide may be applied to the porous support from a solution, they may alternatively be applied by other means such as by vapor deposition, or neat.
Membrane fouling can occur from adhesion of suspended particles, scaling by insoluble salts, and bacterial fouling. While changing the polymer of the membrane may change properties such as the permeability to various ions, the membrane surface energy, or the membrane surface charge, it would also require large changes in membrane fabrication.
Membrane manufacture can be done in a dedicated facility with lines operating in a semi-continuous process. Introducing membranes with new starting materials and membrane coating processes can be time-consuming and expensive. It can be less expensive to make use of existing process lines and materials to make a variety of different composite membranes.
Means for improving the performance of membranes by the addition of constituents to the amine and/or acid halide solutions are described in the literature. For example, U.S. Pat. No. 4,950,404, issued to Chau, describes a method for increasing flux of a composite membrane by adding a polar aprotic solvent and an optional acid acceptor to the aqueous amine solution prior to interfacially polymerizing the amine with a polycarboxylic acid halide. Similarly, U.S. Pat. Nos. 6,024,873; 5,989,426; 5,843,351; 5,733,602; 5,614,099; and 5,576,057 to Hirose, et al. describe the addition of selected alcohols, ethers, ketones, esters, halogenated hydrocarbons, nitrogen-containing compounds and sulfur-containing compounds having a solubility parameter of 8 to 14 (cal/cm3)1/2 to the aqueous amine solution and/or organic acid halide solution prior to interfacial polymerization.
Methods of improving membrane performance by post-treatment are also known. For example, U.S. Pat. No. 5,876,602 to Jons, et al. describes treating a polyamide composite membrane with an aqueous chlorinating agent to improve flux, lower salt passage, and/or increase membrane stability to base. U.S. Pat. No. 5,755,964 to Mickols discloses a process wherein the polyamide discriminating layer is treated with ammonia or selected amines, e.g., butylamine, cyclohexylamine, and 1, 6 hexane diamine. U.S. Pat. No. 4,765,897 to Cadotte discloses the post treatment of a membrane with a strong mineral acid followed by treatment with a rejection enhancing agent.